The present invention relates to a cathode ray tube such as a color picture tube or the like, and particularly, to a cathode ray tube which reduces the deflection power and a leakage magnetic field.
For example, a color picture tube as a cathode ray tube comprises a vacuum envelope which has a substantially rectangular panel, a cylindrical neck, and a funnel positioned between the panel and the neck. The larger diameter end of the funnel is connected with the panel and the smaller diameter end of the funnel is connected with the neck.
A phosphor screen consisting of dot-like or stripe-like three-color phosphor layers which radiate in blue, green, and red is formed on the inner surface of the panel. Opposed to the phosphor screen, a shadow mask having a number of electron beam apertures is provided inside the phosphor screen. In addition, an electron gun for emitting three electron beams is arranged in the neck. A deflection yoke is mounted on the envelope so as to extend from the outside of the smaller diameter portion in the neck side of the funnel to the outside of the neck.
Further, in the color picture tube, three electron beams emitted from the electron gun are deflected in the horizontal and vertical directions by horizontal and vertical deflection magnetic fields generated from the deflection yoke, to horizontally and vertically scan the phosphor screen through the shadow mask. A color image is thus displayed.
A self-convergence inline color picture tube is widely put to practical use as a color picture tube as described above. In this picture tube, the electron gun is of an inline type which emits three electron beams which pass through one same horizontal plane, and the horizontal deflection magnetic field generated from the deflection yoke is of a pin-cushion type while the vertical deflection magnetic field is of a barrel type. Further, the three electron beams arranged in line and emitted from the electron gun are deflected by the horizontal and vertical magnetic fields, to concentrate the three electron beams over the entire phosphor screen without requiring any special correction means.
In this kind of cathode ray tube, reduction of power consumption is a significant problem in view of energy saving. Therefore, it is important for a cathode ray tube to reduce the power consumption of the deflection yoke, and simultaneously, it is desired to reduce leakage of magnetic fields from the deflection yoke.
Specifically, to raise the screen luminance of a cathode ray tube, it is necessary to increase the anode voltage which finally accelerates electron beams. In addition, the deflection frequency must be increased to respond to OA devices such as a HDTV (High Definition Television), a PC (Personal Computer), and the like. However, both the increases of the anode voltage and the deflection frequency lead to an increase in the deflection power.
Meanwhile, in case of an OA device such as a PC which is operated by an operator close to the device, reinforcement is taken with respect to restrictions to a leakage magnetic field which leaks to the outside of the cathode ray tube from the deflection yoke. A countermeasure for such reinforcement is necessary. Conventionally, a method of adding a compensation coil is generally used to reduce the magnetic field that leaks from the deflection yoke. However, if a compensation coil is added, the power consumption is increased.
Generally, in order to reduce the deflection power and the leakage magnetic field in a cathode ray tube, the neck diameter is decreased and the outer diameter of the smaller diameter portion of the funnel where the deflection yoke is installed is also decreased, so that deflection magnetic fields efficiently make effects on electron beams.
However, since electron beams pass close to the inner surface of the smaller diameter portion of the funnel, electron beams extending toward a corner portion of the screen at a maximum deflection angle collide into the inner wall of the smaller diameter portion of the funnel if the neck diameter and the outer diameter of the smaller diameter portion of the funnel are too small. As a result of this, corner portions of the phosphor screen includes a part on which the electron beams cannot reach. If electron beams continue colliding into a part of the inner wall of the smaller diameter portion of the funnel, the temperature of the part increases to melt glass, leading to a risk of implosion. Therefore, in a conventional cathode ray tube, it is difficult to greatly decrease the neck diameter and the outer diameter of the smaller diameter portion of the funnel, to reduce the deflection power.
As a measure for solving the problem as described above, Japanese Patent Application KOKOKU Publication No. 48-34349 suggests the smaller diameter portion of a funnel formed as a pyramid-like cone portion whose cross-section gradually changes from a circular shape to a rectangular shape in a direction toward the panel from the neck side, from the view point that if a rectangular raster is drawn on the phosphor screen, the electron beam passing area in the smaller diameter portion of the funnel on which the deflection yoke is mounted is also substantially rectangular.
If the smaller diameter portion of the funnel is formed to be a pyramid-like cone portion, compared with a normal funnel in which the cross-section of the smaller diameter portion has a substantially circular shape, the diameters in the horizontal and vertical axes are decreased, so that horizontal and vertical deflection coils of the deflection yoke can be arranged closer to courses of the electron beams and the electron beams can be deflected efficiently. The deflection power is therefore reduced.
However, as the cross-section of the cone portion is approximated to a rectangular shape to reduce efficiently the deflection power, the air-pressure withstand strength of the vacuum envelope decreases and the safety is spoiled. Therefore, the shape of the cone portion must be appropriately rounded for practice, and it is therefore difficult to sufficiently reduce the deflection power.
As for the leakage magnetic field, the deflection coil diameter gradually increases from the neck side to the phosphor screen side, and therefore, the magnetic field which leaks toward the phosphor screen extends far. Accordingly, to reduce the leakage of magnetic fields, the diameter of the deflection coil in the side of the phosphor screen must be reduced. Specifically, the cone portion must be shaped to be sufficiently rectangular from the neck side to the phosphor screen side in order to reduce the deflection power and the leakage of magnetic fields.
However, in the vicinity of the end portion of the almost rectangular cone portion, in the phosphor screen side, the cross-sections close to the ends of the horizontal axis (H-axis) and the vertical axis (V-axis) are nearly a flat shape, according to results of analysis of stress calculation, so that these flat shaped portions are deformed in the direction toward the tube axis. As a result, a compressive stress generates in the vicinity of the ends of the horizontal axis (H-axis) and the vertical axis (V-axis), and a tensile stress generates in the vicinity of the ends of the diagonal axes (D-axis) of the cone portion. If the cone portion has a pyramid-like shape, the stress far exceeds 1200 psi which is a standard when designing a general cathode ray tube, so that the cathode ray tube is weak against an external impact and cannot satisfy specifications required for safety.
Also, in case of using a funnel whose cone portion has a pyramid-like shape in a wide angle tube, there can be obtain a cathode ray tube with a practical deflection power. However, since a much greater stress is incurred if the cone portion has a pyramid-like shape as described above, such a cone portion cannot be easily adopted. Finally, to design a tube with a wide deflection angle with use of a funnel whose cone portion has a pyramid-like shape, the cone portion may not be shaped consciously into a pyramid-like shape but should be rounded to some extent in view of the safety, although the reduction efficiency with respect to the deflection power and the leakage of magnetic fields is degraded.
Also, if the cone portion is thus shaped into a pyramid-like shape, costs for components constituting the deflection yoke are increased accordingly. Such a cone portion is not worth while unless it results in an effect of reducing the deflection power and the leakage of magnetic fields, to some extent. It is thus difficult to practice a cathode ray tube having a pyramid-like cone portion.
Meanwhile, for example, Japanese Patent Application KOKAI Publication No. 61-19032 discloses a deflection yoke in which the inner diameter of the core in the vertical direction is reduced in a manner in which a plurality of grooves are formed along the center axis of the inner surface of the core to make the core close to the courses of electron beams as much as possible such that the depths of the grooves decrease as the angles of the grooves with respect to the vertical axis increase, and coil winds of a vertical deflection coil is provided in the grooves.
Also, Japanese Patent Application KOKAI Publication No. 63-241843 discloses a deflection yoke in which the inner diameter of the core in the vertical direction is reduced in a manner in which a plurality of grooves having a substantially equal depth are formed along the center axis such that the inner surface of the core projects in the vicinity of the vertical axis, and coil winds of a vertical deflection coil are provided in the grooves.
Further, Japanese Patent Application KOKAI Publication No. 7-37525 suggests a deflection yoke in which the inner diameter of the core is reduced in a manner in which a vertical deflection coil is shaped to be elliptic along the outer surface of a horizontal deflection coil, and the inner surface of the core is shaped to be elliptic along the outer surface of the vertical deflection coil.
However, every of the deflection yokes described above is installed on a smaller diameter portion of a funnel having a circular lateral cross-section. Therefore, the inner diameter of the core cannot be reduced sufficiently in comparison with a conventional normal deflection yoke, and a great advantage cannot be expected from those deflection yokes. In addition, each of those cores requires a higher manufacturing cost than a conventional normal deflection yoke, resulting in that the costs are increased in spite of its reduced deflection power and it is therefore difficult to put them to practical use.